Femoral component for an implantable hip prosthesis

ABSTRACT

An orthopedic prosthesis for use in a hip replacement surgery includes an implantable stem component. The implantable stem component includes a core and a shell extending over the core. The shell includes a polymeric material and is configured to receive a femoral head component. Metal foam may extend over a portion of the shell.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.13/829,026, which was filed on Mar. 14, 2013, and which is incorporatedherein by reference.

TECHNICAL FIELD

The present disclosure relates generally to orthopaedic prostheses, andparticularly to orthopaedic prostheses for use in hip replacementsurgery.

BACKGROUND

Joint arthroplasty is a well-known surgical procedure by which adiseased and/or damaged natural joint is replaced by a prosthetic joint.The prosthetic joint may include a prosthesis that is implanted into oneor more of the patient's bones. Many hip prostheses include a femoralprosthesis that is implanted into a patient's femur. A femoralprosthesis typically includes an elongated stem component that isreceived in the medullary canal of the patient's femur and aspherically-shaped head component that bears against the patient'sacetabulum or a prosthetic replacement acetabular cup.

Many femoral prostheses are formed from metallic materials or acombination of metallic and polymeric materials. According to Wolff'slaw, a patient's bone tissue will remodel in proportion to the stressapplied it. Because elongated stem components formed from metaltypically have an elastic modulus greater than the elastic modulus ofthe patient's bone, metallic stem components may shield the patient'sbone from stress such that the proximal femoral bone does not remodel toan effective degree, possibly resulting in a loss of support for theimplant and/or implant failures.

SUMMARY

According to one aspect of the disclosure, an orthopaedic hip prosthesisis disclosed. The orthopaedic hip prosthesis includes a femoral headcomponent including a spherical surface shaped to engage a prostheticacetabular component and a femoral stem component. The stem componenthas a first shell including a neck configured to be secured to thefemoral head component and an elongated body extending distally from theneck. The first shell includes a polymeric material. The stem componentalso includes a core that is positioned in the first shell, and the coreis formed from a material having a high tensile strength and a highelastic modulus. The core includes a first core body extending into theneck of the first shell and a second core body extending into theelongated body of the first shell. A second shell extends over aproximal section of the elongated body of the first shell. The secondshell is formed from a metallic foam.

In some embodiments, the first shell may include a shoulder having adistal surface, and the second shell may include a proximal end that isengaged with the distal surface of the shoulder.

In some embodiments, the second core body of the core may have a medialsurface and a lateral surface positioned opposite the medial surface,and the second shell may have a medial surface and a lateral surfacepositioned opposite the medial surface. When the orthopaedic hipprosthesis is viewed in a transverse plane, the first shell and thesecond shell may define a first thickness between a medial-most point ofthe medial surface of the second shell and a medial-most point of themedial surface of the first core body and a second thickness between alateral-most point of the lateral surface of the second shell and alateral-most point of the lateral surface of the first core body. Thefirst thickness may be less than the second thickness.

In some embodiments, when the orthopaedic hip prosthesis is viewed inthe transverse plane, the medial surface of the first core body may beconvex and the lateral surface of the first core body is convex.

In some embodiments, when the orthopaedic hip prosthesis is viewed inthe transverse plane, the medial surface of the first core body may bedefined by a first radius and the lateral surface of the first core bodymay be defined by a second radius that is greater than the first radius.

Additionally, in some embodiments, the transverse plane may be a firsttransverse plane extending through the first shell, the second shell,and the core, and the first shell may have a medial surface and alateral surface positioned opposite the medial surface. When theorthopaedic hip prosthesis is viewed in a second transverse planeextending through the orthopaedic hip prosthesis distal of the secondshell, a third thickness may be defined between a medial-most point ofthe medial surface of the first shell and a medial-most point of themedial surface of the first core body. The third thickness may begreater than the first thickness.

In some embodiments, the femoral head component may include a taperedbore, and the neck of the first shell may include a tapered postconfigured to be received in the tapered bore of the femoral headcomponent.

In some embodiments, the femoral head component may include a bodyincluding the spherical surface. The femoral head component may alsoinclude a polymeric insert positioned in the body, and the insert mayhave the tapered bore defined therein.

In some embodiments, the metallic foam shell may be shaped to engage asurgically-prepared proximal end of a patient's femur. Additionally, thefirst shell may be formed from a metal-polymer composite material. Insome embodiments, the material of the core may be selected from a groupconsisting of a cobalt-chromium alloy and a titanium alloy.

According to another aspect, an orthopaedic hip prosthesis is disclosed.The orthopaedic hip prosthesis includes an implantable distal stemcomponent including a core formed from a material having a high tensilestrength and a high elastic modulus and a shell extending over the core.The shell including a tapered post configured to be received in atapered bore of an implantable head component. The shell includes apolymeric material.

In some embodiments, when the orthopaedic hip prosthesis is viewed in atransverse plane extending through the shell and the core, a firstthickness may be defined between a medial-most point of the shell and amedial-most point of a medial surface of the core, and a secondthickness may be defined between a lateral-most point of the shell and alateral-most point of a lateral surface of the core. The first thicknessmay be less than the second thickness.

In some embodiments, the shell may include a sheath extending over aproximal end of the core and a cover layer extending distally from thesheath. The cover layer may engage only a lateral surface of a distalend of the core.

Additionally, in some embodiments, the shell may be formed from ametal-polymer composite material. In some embodiments, the core may beformed from cobalt-chromium alloy.

In some embodiments, the shell may be a first shell including anelongated body, and the implantable distal stem component may include asecond shell extending over a proximal section of the elongated body.The second shell may be formed from a metallic foam. In someembodiments, the second shell may be formed from titanium. In someembodiments, the second shell is formed from cobalt-chromium alloy.

According to another aspect, an orthopaedic hip prosthesis includes animplantable distal stem component. The stem component includes a firstshell, a core positioned in the first shell, and a second shell. Thefirst shell includes a tapered post configured to be received in atapered bore of an implantable head component and an elongated bodyextending distally from the tapered post. The first shell is also formedfrom a metal-polymer composite material. The elongated body of the firstshell includes a distal section shaped to engage the surgically-preparedproximal end of a patient's femur distal of the second shell.

The core is formed from a material having a high tensile strength and ahigh elastic modulus. The core includes a first core body extending intothe tapered post of the first shell and a second core body extendinginto the elongated body of the first shell. The second shell extendsover a proximal section of the elongated body of the first shell, andthe second shell is formed from a metallic foam and including a porousouter surface shaped to engage a surgically-prepared proximal end of apatient's femur.

BRIEF DESCRIPTION OF THE DRAWINGS

The detailed description particularly refers to the following figures,in which:

FIG. 1 is a perspective view of one embodiment of an implantable hipprosthesis;

FIG. 2 is a cross-sectional elevation view of the implantable hipprosthesis of FIG. 1;

FIG. 3 is a cross-sectional view of the implantable hip prosthesis takenalong the line 3-3 in FIG. 2;

FIG. 4 is a cross-sectional view of the implantable hip prosthesis takenalong the line 4-4 in FIG. 2;

FIG. 5 is a perspective view of another embodiment of the implantablehip prosthesis;

FIG. 6 is a cross-sectional elevation view of the implantable hipprosthesis of FIG. 5;

FIG. 7 is a cross-sectional view of the implantable hip prosthesis takenalong the line 7-7 in FIG. 5; and

FIG. 8 is a cross-sectional view of the implantable hip prosthesis takenalong the line 8-8 in FIG. 5.

DETAILED DESCRIPTION OF THE DRAWINGS

While the concepts of the present disclosure are susceptible to variousmodifications and alternative forms, specific exemplary embodimentsthereof have been shown by way of example in the drawings and willherein be described in detail. It should be understood, however, thatthere is no intent to limit the concepts of the present disclosure tothe particular forms disclosed, but on the contrary, the intention is tocover all modifications, equivalents, and alternatives falling withinthe spirit and scope of the invention as defined by the appended claims.

Terms representing anatomical references, such as anterior, posterior,medial, lateral, superior, inferior, et cetera, may be used throughoutthe specification in reference to the orthopaedic implants or prosthesesand surgical instruments described herein as well as in reference to thepatient's natural anatomy. Such terms have well-understood meanings inboth the study of anatomy and the field of orthopaedics. Use of suchanatomical reference terms in the written description and claims isintended to be consistent with their well-understood meanings unlessnoted otherwise.

Referring to FIG. 1, an orthopaedic prosthesis is illustrativelyembodied as an implantable hip prosthesis 10 for use during a hipreplacement procedure. The implantable hip prosthesis 10 (hereinafterprosthesis 10) includes a head component 12 and an elongated stemcomponent 14 that is configured to be inserted into an intramedullarycanal of a patient's surgically-prepared (e.g., reamed and/or broached)femur. The head component 12 includes a spherical outer surface 16configured to engage a patient's natural acetabulum (not shown) or aprosthetic acetabular cup implanted into the patient's pelvic bone. Thehead component 12 also includes a distal surface 18 having an opening 20defined therein, and an inner wall 164 (see FIG. 2) extends inwardlyfrom the opening 20 to define a bore 22 in the head component 12.

The stem component 14 may be provided in a number of differentconfigurations in order to fit the needs of a given patient's anatomy.In particular, the stem component 14 may be configured in variousdifferent lengths to conform to the patient's anatomy (e.g. a relativelylong stem component 14 for use with a long femur, a relatively shortstem for use with a short femur, et cetera). As shown in FIG. 1, thestem component 14 includes a shell or casing 24 and a shell 26 that issecured to the casing 24. The casing 24 has a shoulder 28 that isengaged with a proximal end 30 of the shell 26. The shoulder 28 definesa collar 32 that has a neck 34 extending proximally and mediallytherefrom.

The neck 34 is configured to be coupled to the head component 12. In theillustrative embodiment, the bore 22 of the head component 12 istapered, and the neck 34 of the stem component 14 includes a taperedpost 40 that is received in the tapered bore 22. When the tapered post40 is seated in the tapered bore 22, the head component 12 is taperlocked onto the stem component 14. It should be appreciated that inother embodiments the neck and the head component may be configured tobe press fit or secured together by other mechanical fastening means.

The casing 24 also includes an elongated body 50 that extends distallyfrom the collar 32. As shown in FIG. 2, the elongated body 50 includes aproximal section 52 that is positioned in the shell 26 and a distalsection 54 that extends outwardly from the distal end 56 of the shell26. In the illustrative embodiment, the neck 34, the collar 32, and theelongated body 50 are formed as a monolithic structure (e.g., a singlemolded or cast part). It should be appreciated that in other embodimentsthe components of the casing 24 (e.g., the neck 34, the collar 32, andthe body 50) may be formed as separate components secured to one anotherby a mechanical fastener (e.g., screw, bolt, taper fit, etc.), adhesive,or other suitable fastener.

As shown in FIG. 2, the casing 24 of the stem component 14 encases aninner core 60. In the illustrative embodiment, the inner core 60 issized and shaped to meet the minimum strength requirements of theprosthesis 10, while the casing 24 and the shell 26 cooperate to definean external geometry of the stem component 14 necessary to fit into theintramedullary canal of the patient's femur. The minimum strength of thecore is determined in accordance with International Organization forStandardization Standard No. 7206-4:2010 “IMPLANTS FOR SURGERY—PARTIALAND TOTAL HIP JOINT PROSTHESES—PART 4: DETERMINATION OF ENDURANCEPROPERTIES AND PERFORMANCE OF STEMMED FEMORAL COMPONENTS” and StandardNo. 7206-6:1992 “IMPLANTS FOR SURGERY—PARTIAL AND TOTAL HIP JOINTPROSTHESES—PART 6: DETERMINATION OF ENDURANCE PROPERTIES OF HEAD ANDNECK REGION OF STEMMED FEMORAL COMPONENTS.” The inner core 60 of thestem component 14 includes a core body 62 having a proximal segment 64and a distal segment 66. The proximal segment 64 extends into the neck34 of the casing 24 to reinforce the tapered post 40, while theelongated distal segment 66 extends into the elongated body 50 toreinforce that section of the casing 24.

The inner core 60 is formed as a monolithic structure (e.g., a singlemolded or cast part). It should be appreciated that in other embodimentsthe components of the core 60 (e.g., the segments 64, 66) may be formedas separate components secured to one another by a mechanical fastener(e.g., screw, bolt, taper fit, etc.), adhesive, or other suitablefastener. In the illustrative embodiment, the inner core 60 is formedfrom an implant grade metallic material having a high tensile strengthand a high elastic modulus (i.e., a high material stiffness). As usedherein, the term “high tensile strength” refers to a tensile strengththat is greater than 650 MPa. Additionally, as used herein, the term“high elastic modulus” refers to an elastic modulus or modulus ofelasticity that is greater than or equal to 100 GPa. In the illustrativeembodiment, the core 60 is formed from cobalt-chromium alloy (“CoCr”)having a minimum ultimate tensile strength of 650 MPa and an elasticmodulus of approximately 195 GPa. It should be appreciated that in otherembodiments the core 60 may be formed any material having a high tensilestrength and a high elastic modulus, including, for example, a titaniumalloy such as Ti-6Al-4V, which has a minimum ultimate tensile strengthof 750 MPa and an elastic modulus of approximately 105 GPa.

The core body 62 of the inner core 60 lies generally in the coronalplane of a patient's body when the prosthesis 10 is secured to thepatient's femur. As shown in FIG. 2, the elongated distal segment 66 ofthe core body 62 includes a medial surface 68 and a lateral surface 70positioned opposite the medial surface 88. When the core 60 is viewed inthe coronal plane, the core elongated distal segment 66 has a thickness72 at its proximal end 74. As shown in FIG. 2, the thickness 72 isdefined between the surfaces 68, 70.

The distal segment 66 of the core body 62 has another thickness 78 atits distal end 76 when the core 60 is viewed in the coronal plane. Thethickness 78, like the thickness 72, is defined between the surfaces 68,70 adjacent to the distal end 76. In the illustrative embodiment, thethickness 78 is less than the thickness 72. In that way, the core body62 tapers to decrease in thickness between the proximal end 74 and thedistal end 76.

As shown in FIGS. 3-4, the medial surface 68 and the lateral surface 70of the distal segment 66 are convex surfaces. As described in greaterdetail below, the medial surface 68 is defined by a radius 80 thatdecreases in magnitude as the medial surface 68 extends from theproximal end 74 of the distal segment 66 to the distal end 76. Thelateral surface 70 is defined by a radius 82 that, like the radius 80,decreases in magnitude as the lateral surface 70 extends from theproximal end 74 of the distal segment 66 to the distal end 76. While theradii 80, 82 decrease in magnitude, the magnitude of the radius 82 ofthe lateral surface 70 is greater than the magnitude of the radius 80 ofthe medial surface 68.

As described above, the casing 24 encases the inner core 60. As shown inFIG. 2, the inner core 60 is completely surrounded by the casing 24. Itshould be appreciated that in other embodiments portions of the innercore 60 may be exposed or extend from the casing 24. In the illustrativeembodiment, the casing 24 is molded over the inner core 60 and is formedfrom a metal polymer composite material having a low elastic modulus. Asused herein, a “low elastic modulus” refers to an elastic modulus ormodulus of elasticity similar to that of a patient's natural femur,which is between about 10 GPa and 20 GPa.

In the illustrative embodiment, the casing 24 is formed from a compositereinforced polymer such as, for example, carbon-fiber reinforcedpolyetheretherketone (“PEEK”). The composite has an elastic modulus ofapproximately 21.5 GPa and an ultimate tensile strength of approximately223 MPa. In that way, the casing 24 has an elastic modulus that iscloser to that of a patient's femur. It should be appreciated that inother embodiments the casing 24 may be formed any composite or polymericmaterial having a low elastic modulus, such as, for example, aglass-filled polymer such as glass-filled PEEK, a non-reinforced polymersuch as neat PEEK, or other reinforced or non-reinforced polymer.

As described above, the stem component 14 of the prosthesis 10 alsoincludes a shell 26 that is secured to the proximal section 52 of thecasing 24. The shell 26 is formed from a metallic foam matrix having alow elastic modulus. In the illustrative embodiment, the shell 26 isformed from a foam matrix of titanium having an elastic modulus ofapproximately 10 GPa and an ultimate tensile strength of the foam matrixof titanium is approximately 35 MPa. In that way, the shell 26 has anelastic modulus that is closer to that of a patient's femur. It shouldbe appreciated that in other embodiments the shell 26 may be formed anymetallic foam matrix having a low elastic modulus, such as, for example,a CoCr foam matrix having an elastic modulus of approximately 19 GPa, aCoCr alloy foam matrix, a titanium foam alloy matrix, or other foammatrix.

As shown in FIG. 1, the casing 24 and the shell 26 have outer surfaces90, 92, respectively. The outer surfaces 90, 92 define the externalgeometry of the prosthesis 10. As such, the outer surfaces 90, 92 engagethe portion of the patient's femur defining the intramedullary canalwhen the prosthesis 10 is inserted into the proximal end of thepatient's surgically-prepared femur. In the illustrative embodiment, theouter surface 92 of the shell 26 is porous to enable bone ingrowthfixation, and the outer surface 90 of the casing 24 is non-porous. Itshould be appreciated that in other embodiments the casing 24 may alsobe porous. It should also be appreciated that in other embodiments thestem component 14 may not include the foam shell 26. In suchembodiments, the outer surface of the casing may define the entireexternal geometry of the stem component 14.

As shown in FIG. 2, the elongated body 50 of the casing 24, the distalsegment 66 of the inner core 60, and the shell 26 cooperate to define alongitudinal axis 94 of the stem component 14. The distal segment 66 ofthe inner core 60 has a longitudinal axis 96 that is offset from theaxis 94 of the stem component 14. In the illustrative embodiment, theaxis 96 is offset in the medial direction from the axis 94 such that thedistal segment 66 of the inner core 60 is biased toward the medial side98 of the stem component 14 and away from the lateral side 100 of thestem component 14. Additionally, in the proximal section 52 of thecasing 24, the thickness of the casing 24 and the shell 26 on thelateral side 100 of the stem component 14 is greater than the thicknessof the casing 24 and the shell 26 on the medial side 98 of the stemcomponent 14.

For example, as shown in FIG. 3, the casing 24 and the shell 26 have acombined lateral thickness 110 and a combined medial thickness 112 whenviewed in a transverse plane extending through the proximal section 52of the casing 24. The lateral thickness 110 is defined between alateral-most point 114 of the lateral surface 70 of the inner core 60and a lateral-most point 116 of an outer surface 92 of the shell 26. Themedial thickness 112 of the casing 24 is defined between a medial-mostpoint 120 of the medial surface 68 of the inner core 60 and amedial-most point 122 of an outer surface 92 of the shell 26. Each ofthe points 114, 116, 120, 122 lies in a common plane, as indicated by animaginary line 126.

As shown in FIG. 3, the lateral thickness 110 is greater than the medialthickness 112. In other words, the combined thickness 110 of the shell26 and the casing 24 on the lateral side 100 of the stem component 14 isgreater than the combined thickness 112 of the shell 26 and the casing24 on the medial side 98 of the stem component 14. In the illustrativeembodiment, the lateral thickness 110 is greater than 5 millimeters, andthe medial thickness 112 is greater than 2 millimeters.

The combined lateral thickness 110 includes a thickness 130 of thecasing 24 and a thickness 132 of the shell 26. As shown in FIG. 3, thethickness 130 is defined between the lateral-most point 114 of thelateral surface 70 of the inner core 60 and a lateral-most point 134 ofa lateral surface 136 of the casing 24. The lateral-most point 134 ofthe inner core 60 lies in the common plane with the other points 114,116, 120, 122. The thickness 132 of the shell 26 is defined between thelateral-most point 134 of the lateral surface 136 of the casing 24 andthe lateral-most point 116 of an outer surface 92 of the shell 26. Inthe illustrative embodiment, the lateral thickness 130 of the casing 24is greater than the lateral thickness 132 of the shell 26.

Similarly, the combined medial thickness 112 includes a thickness 140 ofthe casing 24 and a thickness 142 of the shell 26. As shown in FIG. 3,the thickness 140 is defined between the medial-most point 120 of themedial surface 68 of the inner core 60 and a medial-most point 144 of amedial surface 146 of the casing 24. The medial-most point 144 of theinner core 60 lies in the common plane with the other points 114, 116,120, 122, 134. The thickness 142 of the shell 26 is defined between themedial-most point 144 of the medial surface 146 of the casing 24 and themedial-most point 122 of an outer surface 92 of the shell 26. In theillustrative embodiment, the medial thickness 140 of the casing 24 isgreater than the medial thickness 142 of the shell 26.

In the distal section 54 of the casing 24, the thickness of the casing24 on the lateral side 100 of the stem component 14 is also greater thanthe thickness of the casing 24 on the medial side 98 of the stemcomponent 14. For example, as shown in FIG. 4, casing 24 has a lateralthickness 150 and a medial thickness 152 when viewed in a transverseplane extending through the distal section 54. The lateral thickness 150is defined between a lateral-most point 154 of the lateral surface 70 ofthe inner core 60 and a lateral-most point 156 of the outer surface 90of the casing 24. The medial thickness 152 of the casing 24 is definedbetween a medial-most point 158 of the medial surface 68 of the innercore 60 and a medial-most point 160 of the surface 90 of the casing 24.As shown in FIG. 4, the lateral thickness 150 is greater than 4.5millimeters. In that way, the lateral thickness 150 of the casing 24 isgreater than the lateral thickness 130.

Returning to FIG. 2, the prosthesis 10 also includes a head component 12that may be secured to the stem component 14. As described above, thehead component 12 includes a tapered bore 22 that receives a taperedpost 40 of the stem component 14. In the illustrative embodiment, thetapered bore 22 is defined in an insert 162 formed from a polymericmaterial such as, for example, polyetheretherketone (“PEEK”) orpolyetherketoneketone (“PEKK”). As shown in FIG. 2, the insert 162includes the opening 20 defined in the distal surface 18 and an innerwall 164 extends inwardly from the opening 20 to define the bore 22. Inthat way, the composite tapered post 40 of the stem component 14 engagesthe polymeric insert 162 when the head component 12 is secured to thestem component 14.

The insert 162 is secured to a body 166 of the head component 12 formedfrom an implant grade metallic material such as, for example,cobalt-chromium alloy (“CoCr”) or a titanium alloy such as Ti-6Al-4V. Asshown in FIG. 2, the body 166 has an aperture 168 defined therein, andthe insert 162 is positioned in the aperture 168. In the illustrativeembodiment, the insert 162 is injection molded into the body 166. Inother embodiments, the insert 162 may be formed separately from the body166 and later secured to the body 166 via a locking device such asmechanical fixation. It should be appreciated that in other embodimentsthe head component 12 may be entirely formed from an implant grademetallic material.

In use, the prosthesis 10 is inserted into a proximal end of a patient'ssurgically-prepared femur. The elongated stem component 14 is receivedin the intramedullary canal and the casing 24 and the shell 26 engagethe portion of the patient's femur surrounding the canal. As describedabove, the inner core 60 is sized and shaped to meet the minimumstrength requirements of the prosthesis 10, while the casing 24 and theshell 26 cooperate to provide the stem component 14 with the externalgeometry necessary to fit into the intramedullary canal. The combinationof the high tensile strength/high elastic modulus core 60 with the lowmodulus casing 24 and the low modulus shell 26 results in a reducedstiffness for the stem component 14 such that stress shielding of thepatient's bone is reduced.

Referring now to FIGS. 5-8, another embodiment of an implantable hipprosthesis (hereinafter prosthesis 210) is shown. The prosthesis 210 issimilar to the prosthesis 10 described above in regard to FIGS. 1-4. Assuch, the same reference numbers will be used to identify features ofthe prosthesis 210 that are the same as those included in the prosthesis10. The implantable hip prosthesis 210 (hereinafter prosthesis 210)includes a head component 12 and an elongated stem component 214 that isconfigured to be inserted into an intramedullary canal of a patient'ssurgically-prepared (e.g., reamed and/or broached) femur. As describedabove, the stem component may not include a metallic foam shell securedto the casing. As shown in FIG. 5, the stem component 214 includes acore 220 and a shell or casing 224 secured to the core 220. The stemcomponent 214 does not include a metallic foam shell such that the core220 and the casing 224 cooperate to define the external geometry of thestem component.

The casing 224 includes a neck 34 that is configured to be coupled tothe head component 12. Like the embodiment of FIGS. 1-4, the neck 34includes a tapered post 40 that is received in the tapered bore 22 ofthe head component 12. The casing 224 also includes a sheath 230 thatextends distally from a proximal end 232 attached to the neck 34 and acover layer 234 that extends distally from the sheath 230 to a distalend 236. In the illustrative embodiment, the neck 34, the sheath 230 andthe cover layer 234 are formed as a monolithic structure. It should beappreciated that in other embodiments the components of the casing 224may be formed as separate components. The separate components may besecured to one another by a mechanical fastener (e.g., screw, bolt,taper fit, etc.), adhesive, or other suitable fastener or securedseparately to the core 220.

As shown in FIG. 5, the core 220 is sized and shaped to meet the minimumstrength requirements of the prosthesis 10, while the casing 224 definesan external geometry of the stem component 14 necessary to fit into theintramedullary canal of the patient's femur. An exemplary core isdescribed in U.S. Utility patent application Ser. No. 13/526,032entitled “DUAL MODULUS HIP STEM AND METHOD OF MAKING THE SAME,” which isexpressly incorporated herein by reference. The core 220 is formed as amonolithic structure (e.g., a single molded or cast part). It should beappreciated that in other embodiments the components of the core 220 maybe formed as separate components secured to one another by a mechanicalfastener (e.g., screw, bolt, taper fit, etc.), adhesive, or othersuitable fastener. The core 220 is formed from an implant grade metallicmaterial having a high tensile strength and a high elastic modulus(i.e., a high material stiffness). In the illustrative embodiment, thecore 220 is formed from cobalt-chromium alloy (“CoCr”) having a minimumultimate tensile strength of 650 MPa and an elastic modulus ofapproximately 195 GPa. It should be appreciated that in otherembodiments the core 220 may be formed any material having a hightensile strength and a high elastic modulus, including, for example, atitanium alloy such as Ti-6Al-4V, which has a minimum ultimate tensilestrength of 750 MPa and an elastic modulus of approximately 105 GPa.

In the illustrative embodiment, the casing 224 molded over the core 220.The casing 224 is formed from a composite reinforced polymer such as,for example, carbon-fiber reinforced polyetheretherketone (“PEEK”). Thecomposite has an elastic modulus of approximately 21.5 GPa and anultimate tensile strength of approximately 223 MPa. In that way, thecasing 224 has an elastic modulus that is closer to that of a patient'sfemur. It should be appreciated that in other embodiments the casing 24may be formed any composite or polymeric material having a low elasticmodulus, such as, for example, a glass-filled polymer such asglass-filled PEEK, a non-reinforced polymer such as neat PEEK, or otherreinforced or non-reinforced polymer.

As shown in FIG. 5, the sheath 230 of the casing 224 has an outersurface 240, and the cover layer 234 has an outer surface 242. The outersurfaces 240, 242 define a portion of the external geometry of theprosthesis 10. As such, the outer surfaces 240, 242 engage a portion ofthe patient's femur defining the intramedullary canal when the implant10 is inserted into the proximal end of the patient'ssurgically-prepared femur. In the illustrative embodiment, the outersurface 240 of the sheath 230 is porous to enable bone ingrowthfixation, and the outer surface 242 of the cover layer 234 isnon-porous. It should be appreciated that in other embodiments the coverlayer 234 may also be porous.

As shown in FIG. 6, the core 220 of the stem component 214 includes aproximal core body 244 and an elongated distal core body 246 extendingfrom the proximal core body 244. The proximal core body 244 extends intothe neck 34 of the casing 224 to reinforce the tapered post 40, whilethe elongated distal core body 246 extends through the sheath 230 of thecasing 224. In the illustrative embodiment, the distal core body 246includes a core segment 250 that is positioned in the sheath 230, and acore segment 252 that is positioned distal of the sheath 230. The sheath230 is attached to and encases a medial surface 254 and a lateralsurface 256 of the core segment 252.

The core body 246 (i.e., the core segments 250, 252) and the casing 224(i.e., the sheath 230 and cover layer 234) cooperate to define alongitudinal axis 260 of the stem component 214. The core body 246 has alongitudinal axis 262. As shown in FIG. 6, the axis 262 is offset fromthe axis 260 in the medial direction such that the core body 246 isbiased toward a medial side 98 of the stem component 214 and away fromthe lateral side 100 of the stem component 214. Additionally, thethickness of the casing 224 on the lateral side 100 of the stemcomponent 214 is greater than the thickness of the casing 224 on themedial side 98 of the stem component 214.

For example, as shown in FIG. 7, the sheath 230 has a lateral thickness270 and a medial thickness 272 when viewed in a transverse planeextending through the sheath 230 and the core segment 250. The lateralthickness 270 of the sheath 230 is defined between a lateral-most point274 of the lateral surface 256 of the core segment 250 and alateral-most point 278 of the outer surface 240 of the sheath 230. Themedial thickness 272 of the sheath 230 is defined between a medial-mostpoint 280 of a medial surface 254 of the core segment 250 and amedial-most point 284 of the outer surface 240 of the sheath 230. Eachof the points 274, 278, 280, 284 lies in a common plane, as indicated byan imaginary line 286.

As shown in FIG. 7, the lateral thickness 270 is greater than the medialthickness 272. In other words, the thickness 270 of the casing 224 onthe lateral side 100 of the stem component 214 is greater than thethickness 272 of the casing 224 on the medial side 98 of the stemcomponent 214. In the illustrative embodiment, the lateral thickness 270is greater than 5 millimeters, and the medial thickness 272 is between 2and 4.5 millimeters.

In the illustrative embodiment, the cover layer 234 of the casing 224decreases in thickness as it extends distally along the core segment252. For example, as shown in FIG. 8, the cover layer 234 has a lateralthickness 290 when viewed in a transverse plane that extends through thecover layer 234 and the core segment 252. The lateral thickness 290 isdefined between a lateral-most point 292 of a lateral surface 294 of thecore segment 252 and a lateral-most point 296 of the outer surface 242of the cover layer 234. As shown in FIG. 9, the lateral thickness 290 isgreater than 4.5 millimeters.

In use, the prosthesis 210 is inserted into a proximal end of apatient's surgically-prepared femur. The elongated stem component 214 isreceived in the intramedullary canal and the sheath 230 and the coverlayer 234 of the casing 224 engage the portion of the patient's femursurrounding the canal. As described above, the core 220 is sized andshaped to meet the minimum strength requirements of the prosthesis 210,while the casing 224 is configured to possess the external geometrynecessary to fit into the intramedullary canal. The combination of thehigh tensile strength/high elastic modulus core 220 with the low moduluscasing 224 results in a reduced stiffness for the prosthesis 210 suchthat stress shielding of the patient's bone is reduced.

While the disclosure has been illustrated and described in detail in thedrawings and foregoing description, such an illustration and descriptionis to be considered as exemplary and not restrictive in character, itbeing understood that only illustrative embodiments have been shown anddescribed and that all changes and modifications that come within thespirit of the disclosure are desired to be protected.

There are a plurality of advantages of the present disclosure arisingfrom the various features of the method, apparatus, and system describedherein. It will be noted that alternative embodiments of the method,apparatus, and system of the present disclosure may not include all ofthe features described yet still benefit from at least some of theadvantages of such features. Those of ordinary skill in the art mayreadily devise their own implementations of the method, apparatus, andsystem that incorporate one or more of the features of the presentinvention and fall within the spirit and scope of the present disclosureas defined by the appended claims.

The invention claimed is:
 1. An orthopaedic hip prosthesis, comprising:an implantable distal stem component including: a core formed from amaterial having a high tensile strength and a high elastic modulus, anda shell extending over the core, the shell including (i) a sheathextending over a proximal end of the core, the sheath having a taperedpost configured to be received in a tapered bore of an implantable headcomponent, and (ii) a cover layer extending distally from the sheath,the cover layer engaging only a lateral surface of a distal end of thecore, wherein the shell is formed from a polymeric material, wherein thecore has a proximal end that is positioned in the tapered post of thesheath.
 2. The orthopaedic hip prosthesis of claim 1, wherein when theorthopaedic hip prosthesis is viewed in a transverse plane extendingthrough the shell and the core, (i) a first thickness is defined betweena medial-most point of the shell and a medial-most point of a medialsurface of the core, and (ii) a second thickness is defined between alateral-most point of the shell and a lateral-most point of a lateralsurface of the core, the first thickness being less than the secondthickness.
 3. The orthopaedic hip prosthesis of claim 1, wherein theshell is formed from a metal-polymer composite material.
 4. Theorthopaedic hip prosthesis of claim 1, wherein the core is formed from acobalt-chromium alloy.
 5. The orthopaedic hip prosthesis of claim 1,wherein the core is formed from a titanium alloy.
 6. The orthopaedic hipprosthesis of claim 1, wherein the implantable head component includes abody including a spherical surface, and a polymeric insert positioned inthe body, the insert having the tapered bore defined therein.